Distortionless radio-frequency amplifier



J. D. BOOTH r 2,384,791 DIsToRTIoNLEss RADIO FREQUENCY AMPLIFIER FiledJan. 6, 1942 2 Sheets-Shea?I 1 M0. .m 0. JA,

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l Sept. 18, 1945. .1. n. BooTH DISTORTIONLESS B ADIO FQUENCY AMPLIFIER 2sheets-Sheet 2 Filed Jan. 6*. 1942 lNvlNToR James oolh.

ATToRNEY Patented Sept. 18, 1945 James D. Booth, Catonsville, Md.,assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation oi' Pennsylvania Application January 0,1942, Serial No.425,766

7 Claims.

My invention relates to radio frequency ainpliers and, in particular,relates to circuit conc nections for radio frequency amplifiersemploying remote cut-on' tubes.

In radio frequency amplifiers employing remote cut-of! tubes undernormal operating conditions, it is found when plotting the gain curvefor carrier input versus carrier output that the response is non-linear.This results in a considerable distortion of the modulation envelope ofcarrier wave, which is particularly noticeable on high modulation levelsand high carrier input levels. The distortion is due to the inherentproperty in the operating characteristics of the tubes and not to theimproper design of circuit components. Choosing the operating point at asuitable plate current value, the resultant output becomes decidedlyunsymmetrical as to the modulation envelope. In order to correct thisinherent disadvantage it is proposed, in accordance with this invention,to introduce inverse feed-back of such character which will maintain theaverage plate current of the tube substantially constant over the audiocycle. In other words. prevent plate current variation at the audio rateof modulation. If plate current variation is permitted, the resultingdistortion is equivalent to a partial detection in the radio frequencystages. In radio frequency amplifiers, it is a primary requisite fortrue amplification that no detection or rectification of the modulationcomponent of the signal shall take place.

One object of my invention is, accordingly, to prevent distortion of themodulation envelope of the carrier wave which otherwise is met with inFigure 1 is a general schematic diagram and Fig. 2 a detailed circuitdiagram of a carrier current wire transmission system which embodies aradio frequency amplifier employing the principles of my invention; and

Fig. 3 is a plot of curves useful in explainingA certain features of myinvention.

This application is a continuation-impart of application Serial No.349,526 for Two-way radio transmission, illed August 2, 1940, now PatentNo. 2,310,060, Feb. 2, 1943, and assigned to the Westinghouse Electric &Manufacturing Company of East Pittsburgh, Pennsylvania.

The system illustrated in Figs. 1 and 2, which is used as illustrativeof the employment of my particular invention, is a carrier frequencycom- `munication system which utilizes only a single Another object ofmy invention is to prevent distortion of the modulation envelope of thecarrier wave by radio frequency amplifiers employing remote cut-oiltubes when operating at high g modulation levels.

- stantiallyprevent even partial detection of the modulation signal whenpassing through a radio frequency amplifier employing tubes in theremote cut-oft condition.

Other objects of my invention will become apparent upon reading thefollowing description taken in connection with the drawings, in which:`

channel for simplex operation. A

Systems of the above type may have several communication centers orstations each being equipped with a transmitter and a receiver. Sinceonly a single communication channel is available, the transmitter, aswell as the receiver, must operate on the same carrier frequency. Thispresents a serious problem of preventing interaction between thetransmitter and the receiver located at the same station and at the sametime each of these must be readilyavailable for operation in quicksuccession where communication is carried out between two remotelylocated stations. In other words, when, let us say, at station A theoperator is speaking to station B, the trans-- mitter at A must befunctioning whereas the receiver at A must be inoperative. On the otherhand, at station B the receiver must function and the transmitter mustbe inoperative. At the next instant, when the operator at B wishes toanswer A, the order must be reversed and the receiver at A is to becomeoperative, whereas the transmitter must be silenced, not only withrespect to the modulation comprising the speech of the operator atstation A, but also the carrier frequency, inasmuch as there can be notwo carriers at the same time.

The operation of a system of this type has ergization of differentelements of the circuits. Mechanical relays have the inherentdisadvantage of possessing mechanical inertia. Furthermore, dimcultiesare caused by moving. contacts in chattering and arcing which, insystems of the type herein considered, seriously interfere with smoothoperation.

The major switching in prior systems involved the use of-transferswitches which often had t be manually operated for going over fromtransmitter to receiver operation, and vice versa. In such systems boththe input and output circuits of the transmitter and the receiverterminated at switch contacts. The necessity of having these circuitsalternately connected and broken proved Another advantage offered by thecommunication system in accordance with this invention is that the thehybrid coils for terminating the circuit are dispensed with and circuitsheretofore so terminated can be directly interconnected.

.A further feature of primary importance of the system resides in theincreased exchange rate of communication over a single channel byproviding electronic control circuits having operating time constantsless than the syllabic time of human speech,

The system finds particular usefulness in carrier current communicationwhich employs power lines as the medium for conducting the carrierfrequency current' from one communication center to the other. In suchsystems single frequency operation is particularly advantageous in thatit simplifies the equipment and makes full use of-the limited facilitieswhich a power line offers for conducting carrier frequency current. Forthis reason the system is stated andv described as a carrier currentsystem, although no limitaf tion is intended thereby and there is nodiierence in the general basic organization of the circuits whether thetransmitted high frequency energy is carried over wires or is emittedthrough the ether space.

The power line is indicated by conductors l, 2 and 3 Vwhich may be adirect current line or twophase or three-phase alternating current. Thecarrier frequency is superimposed on one pair of lines shown here as 2and 3 by means of a coupling system employing condensers 4, 5, 6 and 4',E' in series to minimize the breakdown of the high voltage of the powerline. These condensers offer a high impedance to the power frequency anda comparatively low impedance at carrier frequency. In series withcondensers 4, 5 and 6 and terminating to ground is a radio frequencyreactance 1, and similarly in series with condensers 4', 5 and 6', is aradio frequency reactance 1'. The latter are for the purpose ofobtaining a comparatively high impedance to the carrier frequencyA to betransmitted. A line tuning. unit is provided between the couplingcircuit and the transmitter, shown here in block form, to match the lineimpedance with thevoutlputcircuit of the transmitter..` lThe transmitterand receiver are shown in one block diagram, since this inventionpermits permanent interconnection of these units as will be-seen later,and both are permanently tied in with the line tuning unit. Blocks T andT indicate telephone apparatus oonnectedto the transmitter and receiver.

The other units o f Fig. 1, ,each representing a Y communication center,are identical with vthe one ljust described.` Similarcomponents thereofare -marked with identical reference characters.

Referring to Fig. 2, the essential circuit elements of a completetransmitter and receiver are shown here in a simplified form. The novelelectronic control circuits'may be applied 4to any type of receiver andtransmitter. y, For the purpose of simplicity the transmitter circuitcomponents includeall the essentiai elements for a complete operativeunit. Similarly, in the re-l ceiver a simple tuned radio frequencycircuit was selected. There is no limitation intended in showing thesecircuits which can be modified as circumstances may demand to have moreselectivity by including other tuned circuits or by employing a'superheterodyne type receiver. Similarly, in the transmitter othertypes of oscillators or modulators well known in the art may bevemployed to obtain any desired transmission characteristics.

Provision is made for the system to operate as follows:

1. When signals are being transmitted to renderthe receiver inoperativeto full transmitter output voltage prior to and during transmission ofcarrier.

2. Release the receiver block after carrier y transmission has ceasedyet soon enough to receive the beginning of the next part of theincoming conversation.

s 3. Rendering the transmitter inoperative by means of the receivedsignal prior to its arrival` dotted lines represent the receivingapparatus,

those in another square the signal frequency transmission circuit andpart of the control circuit of the transmitter, whereas those in thethird square, the main components of the transmitter. A simple telephonehandset 8 provides for transmission and reception of signals and isconnected bya telephone line comprising conductors 9 and l0 in serieswith the customary microphone voltage source represented by the batteryll to the input circuit of the transmitter and the output circuit of thereceiver. These two circuits therefore are permanently tied together bythe telephone lines 9 and I0. Similarly, the input circuit of thereceiver and the output circuit of the transmitter are permanently tiedtogether by conductors Il and i2 which ultimately terminate in the linetuning unit shown in Fig. 1. It may be mentioned here prior todiscussing the circuit that a permanent interconnection of this typecould not be effected heretofore because of the interaction that atransmitter would have on a receiver, and vice versa, resulting in aserious oscillation of the entire system referred to in telephonepractice as singing A number of relays and switches had to be used totransfer the telephone lines 9 and III and the output-input conductorsII, I2 to the circuits lunder operation. K

The receiver includes in a conventional circuit arrangement the vacuumtubes I3, Il and I5 in which the vacuum tube I 3 performs the functionof a carrier frequency amplifier having. its input circuit between gridI6 and cathode I1 connected to the conductors I I and I2 by a suitablecoupling circuit.

The present invention in distortionless radio frequency amplifiers isembodied in the tube I3 and comprises the combination of resistive,capacitive and inductive bypass elements 25, 25 and'21 in the cathodecircuit of the tube I3. The tube I3 receives modulated carrier inputfrom the transformer I8 which is tuned by condenser I9 and a seriestuned circuit comprising the primary coil 20 .of the transformer I9, aseries capacity 2| and a secondary winding 22 of the input transformer23. The primary winding 24 of the latter connects directly with theconductors II and I2.

' The vacuum tube I3 is shown to be a pentode voltage amplifier derivingoperating potentials from the source shown here by the battery 28.

Referring to the above-mentioned network in the cathode lead, theinductance 21 is a reactor which has a comparatively high impedance atthe modulation frequencies, whereas the ohmic resistance thereof is ofsuch value which will pro'- vlde a suitable voltage drop for the gridbias. I have found it desirable that the inductance 21 be of theiron-core type to readily provide the high inductance required for mostservice, In other words, the grid bias is obtained from the inherentohmic resistance of the choke coil 21. If-the choke coil does not havean ohmic resistance of suitably high value, a resistance may be insertedin series therewith. On the other hand, if the ohmic value of theinductance 21 is too high, resulting in excessive bias for the grid ofthe tube, a, compensating battery may be included between ground and thegrid return so poled as as to provide a positive bias to counteract thenegative bias produced by the high ohmic resistance. Let us assume thatwe have the proper inductance 21 with the necessary ohmic resistancevalue so that no auxiliary bias battery is needed. In parallel with theinductance 21 we have the resistor 25 which is chosen to be of a valueslightly lower than the reactance of the inductance atthe 'lowestmodulation frequency.

` 29 is substantially resistive over the audio frequency range.

Describing the operation of the amplifier, reference is to be had toFig. 3 which illustrates by means of curves the carrier output withrespect to carrier input voltage and also the variation of plate currentin a conventional amplifier. Experimental investigations showed that ifthe plate 'I'he condenser 25 in parallel with the above de- Icomponentof the signal.-

current is maintained substantially constant the carrier output responsecurve looses its undesirable curvature and becomes very nearly linear,within .a wide range of carrier input voltages. The problem, therefore,is to find means whereby the plate current may be effectively heldconstant over the audio cycle. In other words, the averl age platecurrent within the modulation rate of change of voltage of the carriershould be held constant. Referring again toFig. 2, this is accomplishedby the inverse feedback circuit in the cathode of the amplifier tube inthe following manner:

'I'he resistor 25, being a non-reactive element, determines theinstantaneous negative grid voltage and any change of plate current iscounteracted by the bias resulting from the voltage drop across theresistor 25 due to plate current. 'I'he function of the inductance 21 asstated before is to have a, high reactance at audio frequencies actingatthe same time as a shunt with respect to the eil'ective direct currentresistance. The capacitor 25, on the other hand, will not influencel thedirect current resistance value of the combination but will function asa low impedance shunt for the carrier frequency. A better understandingmay be had if we again bear in mind that the 'object is to derive adegenerative feedback voltage only from the modulation frequencyConsequently, a high impedance must be provided for the modulationfrequency and a low impedance for the carrier frequency in the cathodecircuit. The high value of the resistance 25 at any frequency providesthe necessary feedback voltage whereas the condenser provides the lowimpedance at carrier frequency. The function of the inductance 21 is toform a convenient direct current bypass forthe conventional circuit inwhich the operating point is determined solely by the voltage drop inthe cathode circuit. Without the inductance 21, the required highresistance for the resistor would adsary for the grid circuit. Byplacing the inductand 2, batteries were chosen to indicate the variousvoltage sources wherefrom static operating potentials are derived forthe different tubes. In this manner the drawings and the explanation ofthe operation of the system can be greatly simplified. It is to beunderstood, of course, that no limitation is intended thereby, and inpresentday practice all these sources may be replaced by suitablerectified alternating current power supplies. `For the same reason thefilament circuit of the tubes has been omitted for it is wellunderstoodthat these tubes require heating currents which may be derived from anysuitable source.

In continuing with the description, the battery 28 has its negativeterminal connected to the ground terminal side of the cathode I1, and atan lintermediate point suitable voltage is tapped off for the screengrid electrode 29. The anode II) is connected to the Positive terminalofthe battery 29 in series with primary winding 3l of the carrierfrequency transformer 32 which is tuned by condenser t8.' This forms`the output circuit of the tube i3. The secondary winding St oftransformer 32 is connected to ademodanodes 35 and 3S whichare-interconnected with cathode 38 of thisl tube is connected Ato thejimethev load resistance of one diode section, its other terminal beingconnected to the low potential side of the 'secondarywinding 3d oftransformer 32 by I vconductor d and also to the control grid il of thevacuum tube through .conductor d2. A condenser i3 connected betweenconductors t@ and i 39' and in. conjunction'with resistor g'serves aselectrode 33 of tube it. The input circuit ci the control tube i3includes the grid ed, conductor -ulatorcomprisingthe duodiodetube ithaving Hone terminal of the secondary winding 34.1 -The l 'latter is:also tuned by .condenser il. The

tion point of resistors s@ and 39'. y The formeris frequency transformer32, returning through conductor El and conductor @6 to the bias source'battery il and cathode Si. The anode is is also -quency vamplifier ofthe transmitter.

Another interconnection is provided between receiver and transmitter bythe conductor si screen grid'ii'l and the anode it which has a loadresistance i9 by-passed by a condenser E56, where as the former suppliesbias potential being eiectively connected between grid di and cathodeet.

The anode t8 connects also through conductor 5| tothe cathode 52 ofcontrol tube 53. This `will be fully described in connection with thedescription of the control circuit. The other portion of the duodiodetube |25, including cathode 56, may be referred -to as the signal diodeor detector of the system feeding the output amplifier tube 55.' Theoutput connection includes between cathode-Eid and conductor it the loadresistors 5l and 5d in series. The junction point thereof is connectedto the grid 55 of tube 5d through a coupling net- Work includingresistor 59, coupling capacity @El and filter capacity 6| connectedbetween the fjunction vpoint of resistor'59 and condenser li andconductor do.

Between 'the cathode E2 and the'anode t3 of the output amplifier tube 55is the output circuit comprising the primary winding @il of the outputtransformer, the secondary winding 65 of which which connects thecathode t2 of tube bil and the anodeof one of the control tubes of thetransmitter which'wiilbe described later. vThe control tube 53 isaduodiode rectier providing by its space current 'a path to the inputcircuit o the tube i3 for the automatic volume control derived from tubeI5 and for control derived from the transmitter through the conductoriii. The anodes S3 and t3 thereof are interconnected and terminate atthe junction point of the grid return circuit of tube I3 and resistor 95and by-pass condenser St and bias source battery et which supplied theminimum operating bias for the grid i6 of tubel3.

The transmitter circuit comprises the following conventional components.The first signal frequency amplifying-stage including vacuum tube |00followed by modulating stage having vacuum tube 4lillthe oscillationgenerator including vacuum tube |02 and the nal output ampliiier stageincluding vacuum tube |93. The other tubes represent the control circuitand will be described later.

Feeding theinput amplier is the coupling transformer Int, the primarywinding it@ of which connects to the conductors 9 and lil. The secondarywinding |96 forms the input circuit of the tube |00 between grid itl andcathode idd connects to conductors 9 and -||l and the anode.

voltage source in the form of battery G6. Operating bias voltage for theinput circuit of the tube.

is obtained from the battery 6l through control tube load resistor 68which is by-passed for signal frequencies by the condenser 69 and gridresistor "l0, the junction point of the two last-mentioned resistorsbeing connected to the anode li of control tube 12 The controlcircuitincludes vacuum tubes E3 and 12, mentioned before, and vacuum tube i3.

" The anode circuit of the vacuum tube l2 was described in partcomprising the anode'li and load resistance '88. vThe operating sourcetherefore is in series with the bias source battery m9. Ancde and screengrid potentials are derived from the battery |10. The screen gridelectrode lil is bypassed by condenser l2. IThe anode circuit includescoupling transformer ||3, the primary winding ll of which connects fromthe positive terminal of the battery to the anode H5 of the tube |0. Thesecondary winding H5 forms the input circuit to amplifying tube i|l|being con- .nected to the grid il'l thereof at one terminal,

the other half of the battery tb, the negative terminal of which isconnected to the cathode 'lli which is also at ground potential. Theinput to the control tube includes the control grid l5, re-

` sistor 16 and bias source battery l1, returning t0 the cathode lt. Thegrid 'l5 is also coupled directly to the output circuit of tube i3. Thisincludes the resistor l@ and the anode 'i9 of the tube i3 in series withresistor 16, operating source battery 80, returning to the cathode 8|.Resistors 'i6 .and 18 are thereby in series and bypassed by thecondenser 82., A suitable voltage tap of the battery 80 supplies thescreen grid whereas the return terminal connects through the bias supplysource battery to conductor @t which terminates at the anode 'i9 of' thecontrol tube 13. 'I 'he4 output circuit of modulator tube |0| includesthev anode M8, the primary winding ||9 of coupling'transmitter 828i, theanode poten tial source battery |2|and the cathode |22. The secondarywinding |23 of the. transformer iid feeds into the primary winding |26of the modulation transformer |25. The secondary winding |28 thereof isin the input circuit of the nal amplifying tube |08 comprising the gridi'l, resistor |28, the secondaryvwinding |26 and the grid bias battery|29 in series. The positive terminal thereof is connected to groundcompleting the circuit to the cathode |30 of the tube |93 which alsoconnects to ground. The output circuit of the amplifier tube |03includes in series between cathode |30 and anode |3|the plate potentialsource battery |32 and the primary winding |33 of the outputtransformerv |34. A suitable tap ofthe battery |32 connects to thescreen grid electrode |35 which is by-fpassed by condenser |36. Thesecondary winding |31 of the output transformer it". connects to theconductors and I2.

The oscillator stage comprises a screen grid type vacuum tube m2 in aconventional feedback coupling. The cathode |38 thereof is connected toground and the grid circuit includes grid electrode |39, grid loadresistor |40 terminating to ground. The anode circuit includes thesupply source battery |4|, radio frequency choke coil |42 and the anode|43 of theV tube |02. The coupling between anode and grid circuit iseected by the variable inductance |44, one terminal of which connects tothe grid |39 and the other in series with condenser |45 to the anode|43. The oscillator circuit includes also series condenser |46 betweengrid |39 and ground and voltage distribution condensers |41 and |48 inseries between ground and the high potential terminal of the variableinductance |44. The junction point between condensers |41 and |48visutilized in coupling the carrier frequency energy to the grid |21 of thefinal amplifying tube |03 through coupling capacity |49 and conductor|50.

The control circuit has two main branches, each operating on therectified component of the signal frequency energy which is derived fromtwo diode rectifiers and |52 which are energized from the amplifier tube|53. One of the contro1 tubes |54 operates from two energizing sources,one of which is the output of the diode |5|, and the other the output ofthe oscillator |02, whereas the other contro1 tube |55 operates from theoutput of the diode |52.

. Following the circuit, the amplifier tube |53 is energizedsimultaneously with amplifier tube |0| from the secondary winding ||6 ofthe coupling transformer ||3 in that the grid electrode |56 thereofconnects to the grid ||1 of the tube I 0|, and the cathode |51 to thecathode |22. The source 22| is also utilized for anode potential throughthe primary Winding |58 of the transformer |59 connecting to the anode|60. The transformer |59 has two secondary windings |6| and |62. Theformer in a full wave rectifying arrangement connects at both terminalsto the anodes |63 and |63 of the diode |5| and the midtap of thiswinding is returned to the cathode |64 through load resistancepotentiometer il. The unidirectional voltage output is taken from thepotentiometer between the rider |65 and. the cathode terminal. Thisoutput is fed to the grid |51 of the control tube |54 in series withradio frequency choke coil |68 and resistor |69,l the cathode beingconnected to the midtap terminal of the potentiometer |65 through biasbattery i 1| Anode potential for the control tube 54 is obtained fromthe battery |12, the positive terminal of which is grounded and thenegative connected to the cathode |10. Screen grid voltage is taken froma'suitable tap to the screen grid electrode |18 by-*passed by condenser|14. I'he anode |15 connects to the conductor 9| previously mentioned inconnection with the de-l scription of the receiver. The anode circuit ofthe control tube |54 may be traced through the |18. The load resistance|11 is paralleled by condenser |19. The rider |80 of the potentiometer|11 connects to the control grid |8| of the control tube |55, returningtothe cathode |82 thereof in series with grid bias battery |83. Anodepotential for the tube |55 is derived from the battery |84, the negativeterminal of which connects to the cathode |82, whereas the positiveterminal isin series with load resistance |85 'and the anodeY |86.Screen grid voltage is derived from a suitable tap which connects to thescreen grid electrode |81. A portion of the battery voltage is utilizedalso to supply operating voltage to the screen grid electrode 98 of theoscillator tube |02. 'I'his is accomplished by grounding a suitablepoint of the battery |84 and tying the screen i grid electrode 88 to theanode |86 through conductor |88. This completes the description of thecircuit. The various phases of the operation of the receiver andtransmitter as a complete unit shall now be described.

Every communication center of the system includes a complete assembly ofreceiver, transmitter and control circuit as shown in Fig. 2,

forming thereby an operating unit in the chain supplied by thecommunication channel. Let us assume that it is desired to send amessage from this station and the microphone of the handset 8 receivesvoice frequencies. The audio-frequency energy isfed through thetransformer |04 and is amplified by vacuum tube |00, and

'also further amplified by vacuum tube |0I, the

output of which is delivered to transformer |25. This energy thenmodulates the carrier frequency which is impressed on the grid |21 ofthe tube |03 through the condenser |49. The energy is then delivered byconductors l2 and which, as shown in Fig. 1, connect to the line tuningunit through which it is delivered to 'thepower lines 2 and 3.

In addition to this function. audio-frequency energy is also deliveredfrom the transformer ||8 to the grid |56 of control amplifier tube |53.The output thereof is delivered to transformer |59 being rectified byboth diodes |5| and |52. The output voltage of diode |5| appears acrosspotentiometer and is applied to the grid |61 of the tube |54. The latteris normally biased to platev current cutoff by the battery I1|.Simultaneously the output voltage of the diode |52 appears acrosspotentiometer |11 and is applied to the grid |8| of control tube |55.The latter is biased by the battery |83 for normal plate current at nosignal conditions. tween cathode |82 and ground is so adjusted that itis greater than the voltage drop of the tube |55 through load resistance|85 thereby applying a slightly negative voltage to the screen grid 98of the oscillator |02 with respect to its cathode |38 which prevents theoscillator tube from generating carrier frequency energy. However, assoon as signal reaches the transformer |59 the voltage appearing at |11will be vmore negative conductor 9 the space current path of the diodeland cuts oil' the anode current of the tube |55.

Hence the potential between the anode |86 and ground becomes positiveand the screen grid 88 electrode 98 and thereby is prevented from oscil-The voltage bev lation. -The signal amplifier tubes, on the other hand,are in a' standby condition, and as soon as signal frequency appears inthe modulation circuit the negative bias is removed from the screenelectrode and the oscillator becomes instantly operative. As soon asthere is no audiofrequency signal, the' oscillator again becomesinoperative. The timesequence of operation is governed by the constantsof the control circuit, and it is important that these constants be of apredetermined character. This will be further explained as the operationof the circuit is un- 'tube |56 which ows from ground through battery55, resistor 95, diode 53 and anode i715, rel turning to the negativeside of the battery i12.

The voltage produced by the current ow across the resistor 95 appliessuflicient negative bias to the control grid I6 of tube i3 to render theinput amplifier stage of the receiver inoperative to the full voltage ofthe output of the transmitter which will appear across the primaryWinding 2d due to the fact that it is connected also across conductorsll and l2.

The time required to block the receiver to inoperative condition is afunction of the values of resistor 95, condenser t and the plateresistance of the tube ld. The time for actuating the carrier at thetube |53 is a function of the values of the capacity |79, resistor lll,capacity 59 and resistance of the screen grid @5f to the cathode |38.The values of the above-mentioned components are so proportioned thatthe receiver is blocked to inoperative condition prior to the beginningof generation of radio frequency energy by the tube |52.

Recapitulating briefly the operation, as soon -as the microphone isenergized, signal frequency currents will appear in the modulatoramplifier stage, and will also produce through the control tube |53 anddiodes l5l and l52 two sets of control voltages. One is utilized totrigger the oscillator tube into action, the latter being normally in astage of inoperativeness, and theother volt-v age is utilized tocut offthe first amplifying stage of the receiver which is `normally inareceptive state.

The receiving functions operate as follows. Upon impression of carrierfrequency voltage on the. input terminals of the primary 'winding 25 oftransformer 23, the signal is amplified through f the first stageincluding amplifier tube it, and

is delivered to the demodulatortube lf3 and also through the conductord5 to the grid till of the control tube 73. The signal from thedemodulator tube is impressed on the grid 55 of the amplifier tube 55through the capacity from the load resistances 51 and 5d of the diodesection cornprising anode 3 5 and cathode 5d. The other diode sectioncomprising anode `@band cathode 38 pro- 'duces va unidirectional voltagewhich is fed through the resistor 55 b y-passed by capacity iti to thegrid 4|' of the automatic volume control tube l5. 'Ihe operation of thistube is in the conventional manner in that the increased signal voltageproduces an-increase of positive voltage acelerar on the grid 4| causingplate current which is in CFI This tube is nor- V such direction as `toapply a negative voltage to the cathode 52 and thereby control the biason the control grid l5 of the first amplifier tube i3. As stated before,the carrier frequency is also impressed on the grid 84 of tube i3. Thelatter is normally biased to cutoi point by the battery 8l.

l2 is biased by the battery 11 to normal plate current flow which, whileit is maintained, is in such direction through resistor 68 as to biasthe output stage of the receiver, namely, the grid 55 of tube 55 tocutoff condition. Upon carrier frequency appearing on the grid B of tubei3, plate current conduction is initiated which, flowing through theresistor l5, will deliver additional negative bias to tube l2 initiatingcutoff condition, thereby removing the negative bias on the grid 5Ecaused by plate current through resistor 68. Thus, we have aninoperative receiver as to its final output stage until such time as acarrier signal either modulated or unmodulated is applied to the vacuumtube 13. The release o current through resistors 'l5 and 18 is also insuch a direction and magnitude as to apply cuto bias to vacuum tubeslill and |53. The time required to cut off tubes lill and |53 is afunction of the values of capacity 52, and the anode impedance of tube13. The time required to release the cutoff bias from vacuum tube 55 inthe output stage of the receiver for delivery of signal to the telephonereceiver 8 is a, function of the values of resistor 55 and Acapacity 69.The values of the above-.mentioned components are such that the vacuumtubes lill and |53 are cut off to nonoperation prior to the removal ofcutoi bias holding the tube 55 produced by current ow through theresistor 63.

It is seen from the above that the receiver is blocked to inoperativecondition prior to and during the generation of radio frequency energyfrom the transmitter. The latter condition is assured by the coupling ofthe oscillator grid lili) to the grid lE'l of control tube l5@ by meansof the capacity 51 which connects to the junction point of the resistor|69 and choke coil |55, thereby some of the carrier is fed to thecontrol tube |5t in order that the receiver shall be blocked as long ascarrier is generated by the transmitter. Upon cessation of signalfrequency, that is, audio-frequency the carrier stops after which thereceiver is restored to operative condition upon the discharge ofcapacitor 9d through resistor The receiver now is in a stand-bycondition, and

upon impression of a received signal the actuation of fcarrier by thetransmitter is prevented by blocking olf the ktubes ll and l53 prior tothe delivery of the received signal to the telephone 8 from thereceiver. Upon cessation of received carrier the tube 55 will be blockedto inoperative condition by the tube 'l2 prior to or simultaneouslyywith the restoration of normal operating bias to the tubes lili and lS.l

In addition to the timing sequence described "above, the principle oflevel sequence is also ap- The impressed signal causes plate currentsflow from the anode potential battery through' n the resistors 'I6 and18. The other control tube;

ative condition at a signal level equal to or below that required toactuate the ilnal ampliiler stage of the receiver that is tube l5. Itthe precautions oi timing and sequence are applied as described above,it becomes possible to obtain automatic operation controlled entirely bythe normal conversational speech, that is, at less than syllabic time. Aloss of time from the initiation of a speech tone at the input of thetransmitter and the delivery of this tone at the output oi the distantreceiver can be made less than ten milliseconds. This value has beenfound by experiment to be entirely negligible in its adverse effects tothe utility of the system. f

I claim as my invention:

1. An amplier comprising a tube having inductance, a capacitor and aresistor connected in parallel with each other and'carrying the entirecathode current of said tube, a control electrode circuitinterconnecting the control electrode with the terminus of said parallelcircuit elements which is remote from said cathode, and

' an output circuit connected between the anode and said terminus.

2. An amplifier comprising a tube having anode, cathode and at least onecontrol electrode;

means for biasing said electrodes for operation on a portion of theoutput characteristic oi said tube whichis remote from the cut-oil.'point, an

inductance, a.l capacitor and a resistor connected in parallel with eachother and carrying the entire cathode current of said tube, a controlelectrode circuit interconnecting the control electrode with theterminus of said parallel circuit elements which is remote from saidcathode. I

3. An amplifier comprising al tube ,having anode, cathode and at leastone control electrode, means for biasing said electrodes for operationon a portion of the output characteristic oi said tube which is remotefrom the cut-oil! point, an inductance, a capacitor and a resistorconnected in parallel with each other and carrying the entire cathodecurrent of said tube, and an output circarrier current having an anode,cathode and at least one control electrode, an inductance which hasan,impedance high at said modulation frequency, a capacitor and aresistor connected in parallel with each other in the cathode lead ofsaid tube, acontrcl electrode circuit interconnecting the controlelectrode with the terminus of said parallel circuit elements which isremote from said cathode, and an output circuit connected between theanode and said terminus.

5. An amplifier `comprising a tube for modulated carrier current havingan anode, cathode and at least one control electrode, an inductancewhich has an impedance high at said modulation frequency, a capacitorand a, resistor connected in parallel with each other in the cathodelead of said tube, said resistor having an impedance cuit connectedbetween the anode and said slightly lower than that of said inductance,a control electrode circuit interconnecting the control electrode withthe terminus of said parallel circuit elements which is remote from saidcathode, and an output circuit connected between the anode and saidterminus.

6. An amplifier comprising a tube for modulated carrier current havingyan anode, cathode and atleast one control electrode, an inductance eelectrode circuit interconnecting the control electrode with theterminus oi' said parallel circuit elements which is remote from saidcathode, and an output circuit connected between the anode and saidterminus.

.7. lAn ampliiler comprising a tube for modu-` lated carrier currenthaving an anode, cathode and atleast one control electrode, aninductance which has an impedance high at said modulation frequency, acapacitor and a resistor connected in parallel with each other in thecathode lead of said tube, said capacitor and said resistor each havingan impedance which is high bat said modu-- lation frequency, a controlelectrode circuit interconnecting the control electrode with theterminus of said parallel circuit elements whichis remote'from saidcathode, and an output circuit connected between the anode and saidterminus.

JAMES D. BOOTH.

